1. Field of the Invention
The present invention generally relates to the formation of integrated circuits, and, more particularly, to the formation of cap layers of electrically conductive lines connecting circuit elements.
2. Description of the Related Art
Integrated circuits comprise a large number of individual circuit elements, such as, e.g., transistors, capacitors and resistors formed on a substrate. These elements are connected internally by means of electrically conductive lines to form complex circuits, such as memory devices, logic devices and microprocessors. In order to accommodate all the electrically conductive lines required to connect the circuit elements in modern integrated circuits, the electrically conductive lines are arranged in a plurality of levels stacked on top of each other.
The performance of integrated circuits can be improved by increasing the number of functional elements per circuit in order to increase their functionality and/or by increasing the speed of operation of the circuit elements. A reduction of feature sizes allows the formation of a greater number of circuit elements on the same area, hence allowing an extension of the functionality of the circuit, and also reducing signal propagation delays. Thus, an increase in the speed of operation of circuit elements is made possible. In modern integrated circuits, design rules of about 90 nm or less can be applied, and further reductions are planned in the future.
A reduction in the size of circuit elements may entail a corresponding reduction in the size of electrically conductive lines. Reducing the size of electrically conductive lines, however, results in an increase of the current density, i.e., the amperage per cross-sectional area of a current flowing through an electrically conductive line increases. This increased current density increases the likelihood of electromigration occurring.
The term “electromigration” denotes a current-induced transport of atoms in conductors. Electrons moving in an electrical field exchange momentum with the atoms. At high current densities, the momentum imparted to the atoms forms a net force which is high enough to propel atoms away from their sites in the crystal lattice. Thus, the atoms pile up in the direction of electron flow.
Moreover, small electrically conductive lines can be subject to moderately high mechanical stress which can be created, e.g., due to different thermal expansion coefficients of the electrically conductive lines and a surrounding dielectric material. Such stress may relax via a diffusion of atoms in the electrically conductive line. The diffusion of atoms entails a transport of material. This phenomenon is denoted as “stress migration.”
Both electromigration and stress migration can lead to a deformation and, finally, to a failure of electrically conductive lines in an integrated circuit. Hence, in the formation of electrically conductive lines, countermeasures adapted to reduce the likelihood of electromigration and stress migration occurring are frequently taken.
A method of forming electrically conductive lines in an integrated circuit according to the state of the art will now be described with reference to FIGS. 1a-1d. FIG. 1a shows a schematic cross-sectional view of a semiconductor structure 1 in a first stage of a method of forming electrically conductive lines in a semiconductor structure according to the state of the art.
The semiconductor structure 1 comprises a substrate 2. The substrate 2 may comprise circuit elements and electrically conductive lines in lower interconnect levels (not shown) and can be formed by means of known methods, such as deposition, ion implantation, oxidation, etching and photolithography. On the substrate 2, a layer 3 of a dielectric material is deposited. Alternatively, the dielectric material may comprise silicon dioxide (SiO2) or silicon nitride (Si3N4). The layer 3 of dielectric material can be deposited by means of known methods, such as chemical vapor deposition or plasma enhanced chemical vapor deposition. In the layer 3 of dielectric material, trenches 4, 5 are formed, which can be done by means of photolithographic techniques known to persons skilled in the art.
A schematic cross-sectional view of the semiconductor structure 1 in a further stage of the method of forming an electrically conductive line in a semiconductor structure according to the state of the art is shown in FIG. 1b. A diffusion barrier layer 6 is deposited over the semiconductor structure 1. The diffusion barrier layer 6 can comprise tantalum (Ta) or tantalum nitride (TaN) and may be formed by means of sputtering, chemical vapor deposition or plasma enhanced chemical vapor deposition. On the diffusion barrier layer 6, a metal layer 7 is formed. In some examples of methods of forming an electrically conductive line according to the state of the art, the metal layer 7 can comprise copper. The formation of the metal layer 7 when comprising copper may be performed by means of electroplating, which is well known to persons skilled in the art.
FIG. 1c shows a schematic cross-sectional view of the semiconductor structure 1 in yet another stage of the method of forming electrically conductive lines in a semiconductor structure according to the state of the art. A chemical mechanical polishing process is performed. In chemical mechanical polishing, the semiconductor structure 1 is moved relative to a polishing pad. Slurry is supplied to an interface between the semiconductor structure 1 and the polishing pad. The slurry comprises a chemical compound reacting with the material or materials on the surface of the semiconductor structure 1. The reaction product is removed by abrasives contained in the slurry and/or the polishing pad.
In the chemical mechanical polishing process, portions of the metal layer 7 and the diffusion barrier layer 6 outside the trenches 4, 5 are removed. After the chemical mechanical polishing, the trench 4 comprises a first electrically conductive line 7 and the trench 5 comprises a second electrically conductive line 8. Portions of the diffusion barrier layer 6 inside the trenches 4, 5 separate the electrically conductive lines 7, 8 from the layer 3 of dielectric material and prevent a diffusion of metal atoms into the layer 3 of dielectric material. Additionally, the diffusion barrier layer 6 increases an adhesion between the metal and the other portions of the semiconductor structure 1.
Due to the increased adhesion between the metal and other portions of the semiconductor structure 1, the metal can be confined in the trenches 4, 5. Thus, a mobility of metal atoms, in particular a mobility of atoms close to the edge of the electrically conductive lines 7, 8, can be reduced. This may lead to a reduced likelihood of electromigration and stress migration occurring.
It may, however, occur that residues of the barrier layer 6 and/or the metal layer 7 are not removed from the surface of the dielectric layer 3 outside the trenches 4, 5 and remain on the surface of the dielectric layer 3, as indicated by reference numeral 13 in FIG. 1c. 
A schematic cross-sectional view of the semiconductor structure 1 in a further stage of the method of forming electrically conductive lines in a semiconductor structure according to the state of the art is shown in FIG. 1d. A cap layer 9 is formed over the electrically conductive line 4 and the electrically conductive line 5. The cap layer 9 can comprise a metal compound and can be formed by means of electroless deposition. In electroless deposition, the semiconductor structure 1 is inserted into an aqueous plating solution. Solvents in the plating solution undergo a redox reaction with the metal of the electrically conductive lines 4,5. In the redox reaction, the metal compound of the cap layer 9 is formed and deposited on the electrically conductive lines 4, 5. Further products of the chemical reaction pass into a solved state in the plating solution and are thus removed from the semiconductor structure 1. A first portion 10 of the cap layer 9 is formed over the electrically conductive line 4. Similarly, over the electrically conductive line 5, a second portion 11 of the cap layer 9 is formed.
Subsequently, a second layer of a dielectric material (not shown), which may comprise about the same material as the layer 3 of dielectric material, can be formed over the semiconductor structure 1. The cap layer 9 separates the electrically conductive lines 4, 5 from the second layer of dielectric material and prevents a diffusion of the metal of the electrically conductive lines 4, 5 into the second layer of dielectric material. Additionally, the cap layer may improve an adhesion between the metal in the electrically conductive lines 7, 8 and other portions of the semiconductor structure 1. This can help reduce a likelihood of electromigration and stress migration occurring.
A problem of the method of forming electrically conductive lines in an integrated circuit according to the state of the art is that, in the electroless deposition, it may occur that the metal compound is deposited not only on the electrically conductive lines 4, 5, but also on the layer 3 of dielectric material, as indicated by reference numeral 12 in FIG. 1d. The deposition of excess cap layer material 12 can be promoted by residues 13 of the barrier layer 6 and/or the metal layer 7 on the surface of the layer 3 of dielectric material. Such excess material can cause electric shorts between adjacent electrically conductive lines. For example, the excess cap layer material 12 of the cap layer 9 may cause leakage currents between the electrically conductive line 7 and the electrically conductive line 8. Electric shorts between electrically conductive lines can adversely affect the performance of an integrated circuit.
In view of the above problem, there is a need for a method of forming a semiconductor structure that avoids electric shorts caused by a deposition of cap layer material on portions of a layer of a dielectric material between electrically conductive lines.